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Fig. 2.

image

Example magnetograms derived from MDI data (left panel) and HMI data (right panel) on 30 March 2011. The overlying red contours indicate the border of the masked areas determined by a threshold value of 30 G (left panel) and 20 G (right panel), inside which the data points are not used in the travel-time measurements. The MDI data on this day shown here is only for comparison since HMI data are used instead of MDI data after 1 May 2010 in this work.

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